This was something I have been working on as a way to explore the possibilities of procedural textures in Cycles. This is all 100% procedural, so no external images were used as part of this Material.
I went through many tests before I reached a result I was happy with. Because it is a procedural Material, you can adjust it as you like.
I used an asset from tuqueque's b.m.p.s' Material Preview Scene, which can be found here: http://blenderartists.org/forum/showthread.php?151903-b-m-p-s-1-5!